1. Field of the Invention
The present invention relates generally to an image sensing apparatus, and more particularly, to an image sensing apparatus such as a video camera having an automatic focusing function for automatically matching the focus relative to an object in response to a video signal obtained from an image sensor.
2. Description of the Background Art
Conventionally, in an automatic focusing apparatus used in an image sensing apparatus such as a video camera, an approach utilizing a video signal itself obtained from an image sensor for evaluating a state in which the focus is controlled has been developed. More specifically, in such an approach, a video signal obtained from an image sensor such as a CCD (charge coupled device) is passed through a high-pass filter (HPF) or a band-pass filter (BPF), to detect a high frequency component thereof as a focus evaluating value every constant time period, for example, every one field, and control a relative position of a lens and the image sensor such that the focus evaluating value reaches a peak. In this approach, an area for detecting the focus evaluating value in an image sensed picture, i.e, a focusing area is generally fixed as an area of constant size in the center of a picture.
Meanwhile, considering a case in which this focusing area is set large, if images of objects at different distances from a lens included in one picture are sensed or the background of the object has a high contrast, that is, has a large high frequency component, it is not determined which object is brought into focus. Thus, an object of interest to a user is not always brought into focus.
In order to prevent such a situation, it is considered that the focusing area is set small to bring only an object in the center of the picture into focus. However, the object frequently moves outside of the focusing area due to unintentional movement of the hands of the user and slight movement of the object, which is liable to cause a malfunction of automatic focusing.
As described in the foregoing, large and small focusing areas have both advantages and disadvantages, respectively. As means for eliminating the disadvantages, a technique has been proposed of providing large and small focusing areas and setting a prescribed level of a focus evaluating value in each of the areas. More specifically, according to such a technique, even if an object moves outside of the small focusing area so that the focus evaluating value in this area attains the prescribed level or less, an auto-focus operation is carried out based on the focus evaluating value in the large focusing area provided that the focus evaluating value in the large focusing area is at the prescribed level or more. Thus, a technique of switching focusing areas is disclosed in, for example, Japanese Patent Laying-Open Nos. 17418/1988, 182704/1987 and 183877/1985. More specifically, according to an in-focus state detecting apparatus disclosed in Japanese Patent Laying-Open No. 17418/1988, a large focusing area is selected after determining that the focus can not be detected in a small focusing area and then, detection of the focus is repeated in the large focusing area. In addition, according to an automatic focusing apparatus disclosed in Japanese Patent Laying-Open No. 182704/1987, a focusing area is enlarged in order to increase a focus evaluating value at the time of a low contrast to stabilize an automatic focusing operation. Furthermore, according to a focus detecting apparatus disclosed in Japanese Patent Laying-Open No. 183877/1985, focusing areas are switched depending on the focal length of an image sensing optical system.
According to the above described conventional techniques, the prescribed level of the focus evaluating value in each of the large and small focusing areas is a fixed value previously set. Therefore, a relative ratio of a peak value (a value in an in-focus state) of the focus evaluating value to the prescribed level which is the fixed value varies depending on the object. As a result, in the case of a high-contrast object, unless the object is considerably spaced apart from the small focusing area, the focus evaluating value does not attain the prescribed level or less so that the large focusing area is not selected. On the other hand, in the case of a low-contrast object, if the object moves slightly outside of the small focusing area, the large focusing area is immediately selected, so that there occurs a difference between movement of the object and timing of switching focusing areas depending on the object.
Additionally, if and when the background of the object has a high contrast, the focus evaluating value is large even in a significantly defocused state. If the focus evaluating value is beyond the prescribed level in the small focusing area, a malfunction occurs. More specifically, even if the object completely moves outside of the small focusing area, the large focusing area is not selected. In addition, if the object moves outside of the large focusing area, a new peak detecting operation is not carried out.
On the other hand, in the conventional automatic focusing apparatus, a focus evaluating value may have no distinct peak during the auto-focus operation. In such a case, a position where a focus evaluating value in a fixed focusing area is slightly projected is considered as an in-focus position, a lens being fixed therein. Thus, in the conventional automatic focusing apparatus, the peak of the focus evaluating value due to noises or the like may be erroneously judged to be the in-focus position, so that a malfunction of automatic focusing is liable to occur.
On the other hand, in an image sensing apparatus such as a video camera, control of a level of a video signal obtained from an image sensor, i.e., exposure control by, for example, adjusting a diaphragm and adjusting the amplification gain, together with the above described focus control, is a very important subject. Conventionally, a video camera has been put into practice which has a function of automatically matching exposure, i.e., an automatic iris function by adjusting an optical diaphragm and the amplification factor of an amplifier for amplifying a level of a video signal obtained from an image sensor based on the average of luminance levels of the video signal and a level of a peak value.
However, such a conventional automatic iris system has disadvantages. For example, if a high luminance portion such as a light source exists in a picture, the diaphragm is driven in the closing direction so that the gain of the entire picture is decreased, whereby a main object is insufficiently bright. On the other hand, if the background is very dark, the diaphragm is driven in the opening direction so that the gain of the entire picture is increased, whereby the main object becomes too bright.
A method of eliminating the disadvantages is disclosed in, for example, Japanese Patent Laying-Open No. 110369/1987. According to such a technique, an optical stop-down mechanism is driven to control the amount of light incident on an image sensor such that a luminance level of the entire image sensed picture coincides with a reference level, and a luminance level in a central area of a picture is weighted, as compared with a luminance level in a peripheral area thereof to control the amplification gain of a video signal considering the central area as a priority area, thereby to decrease the effect of an abnormal luminance portion such as a light source which exists in the peripheral area on exposure control.
Meanwhile, if and when the above described automatic focusing apparatus and exposure control apparatus are equipped with one video camera, the same area can be set as a focusing area where an object exists and a priority area where an object exists. However, considering a case in which large and small focusing areas are provided and switched as described above, when the priority area is fixed in, for example, the smaller focusing area and the object laterally moves to be outside of the small focusing area, the larger focusing area is selected so that an in-focus operation is most suitably carried out. However, as for exposure control, only exposure in the smaller focusing area where no object exists becomes most suitable. As a result, overexposure or underexposure is liable to be obtained with respect to the object which moves outside of the smaller focusing area.